1. Field of the Invention
The present invention relates to a method and an apparatus for the in situ purification by biofiltration and aeration, of water within water bodies such as waste water treatment plants, lakes, bays, ponds or lagoons.
The invention also relates to a method and apparatus of the reduction of sludge within such waterbodies.
2. Brief Description of the Prior Art
Major goals of waste water treatment are to remove organic or nitrogenous pollutants as well as excessive concentrations of metals from the water, and to assure adequate levels of oxygen. Associated with the improvement is the goal to reduce sludge accumulation equally as part of the waste water treatment process or within water bodies.
Hitherto, the biological purification of water by aerobic processes, has been accomplished by providing aeration either within the water column or by passing water through a physical substrate. In the first case, water purifying micro-organisms are supposed to appear spontaneously within the aerated water, to biodegrade the pollution. In the second case, the micro-organisms are supposed to also appear spontaneously and develop on the physical substrate as aerated water flows through the physical substat. In both situations the micro-organisms are not only supposed to appear but they are also supposed to be in the optimal physiological condition. Biological filters provide physical surfaces on which the micro-organisms can develop and in this manner biological filters increase the numbers of micro-organisms available for the treatment, in comparison with those in the water alone.
The efficiency of the biological purification is known to be function of various key factors including:
the availability of a balanced community of micro-organisms and other organisms capable of biodegrading or, at least, bioaccumulating the target pollution, PA1 the quantity of micro- and other organisms in the above community, PA1 the physiological condition of the micro-organisms in the above community, PA1 the surface area of the physical substrate available for the growth of the micro-organisms, PA1 the nature of the substrate to allow colonization by the micro-organisms, PA1 the flow rate through the biological filter to assure the adequate supply of food to the micro-organisms, to assure that the pollution is removed by the biological filter faster than it enters the water body, PA1 the open area within the biological filter to minimize clogging and frequency of cleaning. PA1 Epilimnetic aeration--this method does nothing to improve conditions in the hypolimnion or at the soil/water interface, which is often a or the major source of pollutants entering the water column. PA1 Bottom mounted aeration--this method destratifies the water column making oxygen levels and temperature uniform. The temperature of the destratified lake usually is closer to that of the epilimnion than to that of the hypolimnion. Thus the lake tends to be warmed. PA1 Hypolimnetic aeration--this method brings oxygen to the hypolimnion without destratifying the water column. A major advantage of this method is that different temperature layers are maintained allowing the development of species preferring cold water in the hypolimnion (e.g. trout) and species preferring warmer conditions in the epilimnion (e.g. bass). PA1 The air diffusers are placed directly below the biological filter. This method entrains anoxic or poorly oxygenated water, as is the goal of an aeration device. It is stated in the patent that the device is designed to be placed in areas with high oxygen demand. However, the placement of the diffusers directly below the porous material does not allow adequate contact time for oxygen transfer. Poorly oxygenated water entering a biological filter is undesirable as it can limit biological activity. PA1 Air bubbles entering the biological filter will coalesce, due to contact with the material, to form larger bubbles. The larger bubbles will have reduced surface to volume ratios and will decrease gas transfer. PA1 Even though the device is described as an aerator, very little actual oxygen addition can be anticipated, because the coalescence of the bubbles as they pass through the biofilter will result in very large bubbles leaving the biofilter and rising to the surface. Such bubbles have very little value in terms of oxygen addition. PA1 The aeration device is not designed to take advantage of the technique of bringing subsurface water, particularly that from the hypolimnion, which is poor in oxygen, in contact with the surface. It is at the surface that water can be inexpensively aerated by being placed in contact with the atmosphere, which contains 20% oxygen. The movement of air through the biofilter will cause the bubbles to coalesce resulting in fewer large bubbles rising to the surface from the biofilter. This random distribution of large bubbles is not efficient for water movement. The large bubbles will move through the water without entraining water to the surface. PA1 A relatively large amount of air must be used to move the water and oxygenate the biofilter because of the above design deficiencies. PA1 The ability to preinoculate the biofilter with communities of micro-organisms specifically chosen for their ability to biodegrade completely the target pollutants to carbon dioxide and water or remove the target pollutants from the water. PA1 The ability to incorporate plants and higher life forms to optimize the overall water quality improvement. PA1 The ability to separate the processes of using air to move the water from the bottom to the surface, aeration, and causing the water to pass through the biofilter. Water flow per quantity of air injected can be optimized if the turbulence and friction caused by passing the air and water through the biofilter is eliminated. This technique would also allow control of the water flow so that water passes through the biofilter with sufficient speed and oxygen content to provide a net gain in oxygen within the lake. PA1 The ability to provide supplemental aeration, with a counter current flow, within the treatment device. PA1 The ability to entrain water, from within the water body, and bring it in contact with the surface and atmospheric oxygen. PA1 The ability to aerate the water from the hypolimnion and return it to this strata without destratifying the lake and raising the temperature of the hypolimnion. PA1 The ability to entrain and digest sediments within the device without distributing the sediments throughout the water body. PA1 The ability to remove BOD, nitrogenous wastes, organic sludge and metals from the water body. PA1 The ability to construct the device in two sections, the biofilter and the enclosure so that discrete sections of the water column or water body can be treated. PA1 The ability to use random packing materials, which have opposed surfaces less than one inch apart. PA1 air used in an airlift to bring the water to the submerged chamber; PA1 air diffused from the surface as the moving water is brought in contact with the atmosphere; and PA1 finally air optionally diffused within the deeper portion of the submerged chamber so that the rising bubbles operate in a counter current manner to the water flow providing efficient gas exchange.
A useful biofilter should also be simple to use and cost-effective.
Hitherto, biological water waste water purification has been accomplished by the use of water based systems such as activated sludge or oxidation or facultative lagoons. Alternatively, fixed film systems such as trickling or rotating biological filters have been used. Expansion of water based systems to meet increasing pollutional loads usually results in the costly expansion of the waste water treatment plant rather than increasing the treatment capacity of the existing structures. Decreasing budgets often associated with limited space have created a demand for ways to increase treatment efficiency cost-effectively.
Hitherto, the combination of aeration and biological filtration has been applied to lakes and other water bodies, although of use of aeration alone is more common in lakes. In temperate climates, lakes and other water bodies generally stratify in summer, resulting in the formation of three distinct layers: the epilimnion--that part of the water column in contact with the surface, the metalimnion--a transition zone, and the hypolimnion--that part of the water column in contact with the bottom of the water body. The epilimnion generally remains oxygenated due to contact with the atmosphere at the surface and is the warmest part of the water body. The hypolimnion, by contrast in eutrophic water bodies, usually becomes anoxic resulting in the loss of higher life forms (fishes and benthic organisms), the liberation of pollutants from the sediments and the rapid accumulation of organic sediments and thus the filling in of the lake. The hypolimnion contains the coldest water in the lake, during summer. The liberated pollutants are subsequently distributed throughout the lake when the layers in the lake are mixed during the spring and fall turnover periods; this then results in increases in algal growth within the epilimnion. Lake aeration which assures the presence of oxygen at the soil/water interface will minimize the release of nutrients from the sediments, allow the development of populations of higher life forms (fishes and benthic organisms) and accelerate the decomposition of organic sediments.
There are three basic approaches to lake or water body aeration:
Bottom mounted aeration entrains water from the bottom to the surface. This technique, when applied to a lake or water body and designed to achieve destratification, is very efficient, because the bulk of the aeration is accomplished by diffusion from the atmosphere. The actual oxygen transfer from the injection of the air into the water is of limited value for several reasons including declining hydrostatic pressure, decreasing oxygen content of the bubbles and increasing oxygen content of the water, which is moving in parallel with the rising bubbles. These problems are particularly problematic in hypolimnetic aeration systems where water from the bottom is moved to the surface by aeration and then returned to depth to avoid destratification of the water column. The inefficiencies, mentioned above, often result in excessive energy being required to achieve the desired effect of increasing oxygen concentration within the hypolimnion.
Aeration efficiency is a function, in part, of the bubble size, the contact time of the bubble with the water and the oxygen gradient between the bubble and the water. Counter current systems are particularly advantageous for increasing the efficiency of gas exchange. The principle of the counter current is that water and air or oxygen supplied travel in opposite directions so that as the water becomes oxygenated it contacts air with increasing oxygen concentration. An additional benefit can be achieved if water flows downward so as to slow the rise of the gas bubbles, thus increasing the retention time of the bubbles within the water column.
In Canadian patent No. 747,976, there is disclosed an aeration device which combines aeration with biological filtration. This device includes a porous mass of material having a large surface area on which biological material or slime is allowed to grow. The device is submerged, in a body of water, and water is caused to flow through the porous material by injection of air below the porous material. The air rises through the porous material entraining water with it bringing water and oxygen in contact with the slime of the biological filter. Even though this device offers the theoretical potential to combine aeration and biofiltration, within a lake, there are several serious conceptual limitations of Canadian patent No. 747,976.
Biological growth on the biofilter is an uncontrolled process. There is no mention in this patent of the need to inoculate, fix or immobilize desired communities of water purifying micro-organisms, and bacterial augmentation has been shown to be able to improve water purification.
There is no suggestion in this patent to use plants to enhance water purification, even though there is a growing wealth of information on the benefits and potential of including plants in the treatment process. Aeration efficiency is also very low with this patented device for many reasons:
With the device of this patent, it is further very difficult to maintain the biological floc on the porous material designed for this purpose, because of the scrubbing action of the air bubbles flowing through the filter media. Injection of air bubbles under submerged biofilters is a standard mechanism used to remove growth accumulations. The continual injection of air bubbles, particularly coalescing ones, will provide a continual scrubbing and removal of the biological growth from the filter media.
Moreover, no mechanism is included to provide hypolimnetic aeration, that is aeration or even water movement without destratification. The biofilter is the only component of the unit; there is no separate flotation platform to avoid generalized destratification.
The patent discloses that the device will help reduce biochemical oxygen demand (BOD), but no claims are made or method shown to reduce sludge accumulations or metals within a water body.
No mechanism is included to place the device in close proximity t,o sediments without introducing considerable suspended solids throughout the water column.
The patent also discloses that the filter media will have opposed surfaces at least one inch apart. This will eliminate the potential for use of random packing materials with smaller openings of products used for bacterial immobilization such as those of W. R. Grace or J. Manville.
A more efficient system would be based on the following concepts: